Brief overview of each
After a pretty fine season at catcher in 2015 where Cervelli stayed pretty healthy and hit for average, got on base and was a pretty good defender the Pirates rewarded him with a three year extension worth $31M through 2019. In 2016 he only played 101 games and in 2017 he was shut down due to injury and only managed 81 games, with a handful of those only being for pinch hitting. Along with the reduced time his bat has falling off along with his defense. Neither extremely poor but certainly not what you would expect from someone making 9M this season. Average at best is how I would describe his performance the past two season.

Elias Diaz has become a nice addition to the roster. His bat hasn't gotten there yet but his value is for his defense. Once his bat comes around to above the Mandoza line he is at worst a solid back up. Should he start hitting closer to league average (.255/.325/.427) then he should be a fair starter on a team that doesn't have a catcher like Posey or Molina. He is smart, has a rocket of an arm that is accurate and seems to work with many of our young pitchers well.

Chris Stewart has been around with 6 different teams in 11 years. Picked up to back up Russell Martin and then Cervelli he has done a generally fine job. The past couple seasons have seen his bat drop off but as a back up he has done well and seems to handle both pitchers and umpires fairly well. Chris is signed through 2018 team option that probably should be picked up at $1.5M even if he is traded shortly after.

Jacob Stallings is the Pirates depth piece only seeing a handful of games since his debut in 2016. pretty fair defensively and was raking it at the plate in AAA Indianapolis in 2017.

My opinion
It is my opinion that Cervelli should be traded. His value is pretty low right now with injury and poor performance but if the team can get a couple mid grade prospects for him they should do so. Now professional teams usually keep insurance on players so if they get injured they get reimbursed for some of the players pay... that isn't too much of a concern, but if the team wants to move forward and appear to compete they need a steady team out their daily. Cervelli has had the opportunity to play 486 games as a pirate (three full seasons) but has missed 174 of them, including rest days.

Elias should take over the starting catching duties permanently if not free agents are picked up ( i'll get to that in a minute). We haven't gotten much in the way of hitting from the catching position the past couple seasons so this really isn't a loss on the offense, however Elias is superior at defense and is far cheaper. All the team will loose is Cervelli's OBP which was still pretty good and the childish look on his face after taking a "bump" that all catchers deal with.

If the team does as I suggest and Cervelli is off loaded in the off season, Stewart is very necessary to the team. If Cervelli is kept and the Team is fairly comfortable with his health then his option picked up and traded after spring. Stewart doesn't have a lot of value but a team hit with an injury bug could be willing to part with a mid range pen arm or fringe prospect(s) for a more than capable backup catcher.

Stallings is more of a career Minors player that provides depth. Should he leave the organization it wouldn't be the worst thing, however if possible he should be kept to see if that bat he showed in AAA during the 2017 season is for real.The offseason / draft

There is really only one catcher of note that is a free agent after 2017 and that is Jonathan Lucroy. As a Pirate fan I loved to hate this guy and the only reason was is that he was a Brewer. Well since he went to Texas and now Walker plays for the Brewers I don't mind him so much. Since going to the AL he has tanked and hard. Now Back in the NL at Colorado he has done much better. I think it is safe to say he is looking for an NL team next season unless he is looking for a place he can DL to save his legs. However even with Lucroy's down performance in the AL, he will still cost too much. I would speculate he gets 2y/40m with a vesting option or maybe 3y/55m but only from an NL team. I wouldn't bet on the Pirates being that team, but if they are able to trade Cervelli and sign Lucroy to a 2y/30-40m deal it would be a huge win for the organization. (both are unlikely)

Any significant trade (Polanco, Cutch, Cole, Harrison, Freese, Cervelli, etc) A catcher should be coming back to the team. Even a fringe prospect. Of course Pitching is priority but after Elias the Pirates have few options in the minors.

In the '18 Draft the Pirates should be looking for catcher first and take the best available catcher in the first or second round. I'm not sure

The To Long; Didn't Read (TL;DR) version, trade Cervelli if you can. If Cervelli is gone make a huge push for Lucroy but if it doesn't happen use Elias as starter and it would still probably be an overall upgrade at catcher. Stewart backing up and Stallings for depth.